Watts anti-syphon valve needs replacing - with what?

Hi, outside of replacing sprinkler heads and broken pipes, I'm a newbie at irrigation.
I've a small garden sprinkler system which has a Watts 1" 800M4QT anti-syphon valve which suddenly failed after 15 years - continually leaking a large volume of water through the bonnet. I tried resetting the internal parts but it still fails. I realize there are service kits but this valve is old and is in bad shape. I'd like to replace it. I see there are other anti-syphon valves out there that are considerably cheaper than the Watts - and my budget is VERY tight. For example, there's a 1" manual brass valve by Champion or Orbit - can I just swap this into my system instead of the Watts? How do I know what my system needs in terms of an anti-syphon device?

If there are ANY valves after the backflow preventer, such as zone valves, you CANNOT use the Champion or any similar valve. That valve can ONLY be used as the control valve for a section in place of a solenoid valve. Most pressure vacuum breaker valves, which is the minimum grade you can use, will cost about the same.

The Watts PVB may not be as old as you think. The do weather. The model you have in my opinion is one of the better ones made. Slightly more expensive than the Febco or Wilkins, but in the long run, better. The repair kit for the upper part (includes the poppet float, the bonnet o-ring and the bonnet spring) runs around $40 and very possible will take care of the problem. Do carefully check the bonnet o-ring first.
Much less likely is a crack in the bonnet.
Also, pay attention to your start up procedure. The valve allowing water into the PVB should be turned on rather briskly to reset the backflow. If the backflow is only partially recharged with pressure on the lawn side and the source side, you can have a "chattering" of the valve. The metal cross piece in the backflow can bend at that time causing problems.